A certain reaction has this form. aA->bB. At a particular temperature and [A]0=2.00 x 10^-2 Molar, concentration versus time data were collected for this reaction and a plot of In[A]t
versus time resulted in a straight line with a slope value of -2.97 x 10^-2 min^-1
How much time is required for the concentration of A to decrease to 2.50 x 10^-3 Molar?
OA 70.03 minutes
OB. 0.0096 minutes
OC 0.589 minutes
OD 29.70 minutes
OE. 100 minutes

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Explanation:

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.